VII JOINTS OR ARTICULATIONS 319 



are connected together in such a manner as to form strong 

 and definitely-arranged joints or articulations. 



Joints may be classified into imjDerfect and perfect. 



(a) Imperfect joints are those in which the conjoined 

 levers (bones or cartilages) present no smooth surfaces, 

 capable of rotatory motion, to one another, but are con- 

 nected by continuous cartilages, or ligaments, and have 

 only so much mobility as is permitted by the flexibility of 

 the joining substance. 



Examples of such joints as these are to be met with in 

 the vertebral column — the flat surfaces of the bodies of 

 the vertebrse being connected together by thick plates of 

 very flexible fibro-cartilage, which confer upon the whole 

 column considerable play and springiness, and yet prevent 

 any great amount of motion between the several vertebrae. 

 In the pelvis (see Plate, Fig. VI. and Fig. 4), the pubic 

 bones are united to each other in front, and the iliac 

 bones to the sacrum behind, by fibrous or cartilaginous 

 tissue, which allows of only a slight play, and so gives the 

 pelvis a little more elasticity than it would have if it were 

 all one bone. 



(6) In all perfect joints, the opposed bony surfaces 

 which move upon one another are covered with cartilage, 

 and between them is placed a sort of sac, which lines 

 these cartilages, and, to a certain extent, forms the side 

 walls of the joint ; and which, secreting a small quantity 

 of viscid, lubricating fluid — the synovia — is called a 

 synovial membrane. 



The opposed surfaces of these ariicnlar cartilages, as 

 they are called, may be spheroidal, cylindrical, or pulley- 

 shaped ; and the convexities of the one answer, more or 

 less completely, to the concavities of the other. 



Sometimes, the two articular cartilages do not come 

 directly into contact, but are separated by independent 

 plates of cartilage, which are termed inter-articular. The 

 opposite faces of these inter-articular cartilages are fitted 

 to receive the faces of the proper articular cartilages. 



